Add method in TelecomManager to return line1Number for a PhoneAccount

Since we do not want to expose SubIds in the app layer, we need a
wrapper to extra the subId from a PhoneAccount and return the
value of TelephonyManager#line1Number.
Also update error strings to be more consistent.

Bug: 19087382
Change-Id: Ie5a474d45336921d1007b36a4d26cd4da68f8b91